Aluminum and process applicable thereto



United States Patent 9 i ALUMINUM AND PROCESS APPLICABLE THERETO Carlo Sonniuo, Brentwood, Mo., assignor, by mesne assignments, to Koenig and Pope, St. Louis, Mo., a partnership No Drawing. Application February 25, 1954, Serial No. 412,651

14 Claims. (Cl. 204-65) This invention relates to the anodic treatment of aluminum and more particularly to such anodic treatment whereby a flexible, corrosion-resistant, long wearing protective coating is produced on the surfaces of the aluminum.

Briefly, the invention is directed to the method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath containing chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the bath being substantially free of combined and free sulfuric acid, and thereafter subjecting the aluminum to the action of steam. The invention is also directed to novel aluminum products obtained by this method and novel bath compositions employed in the practice of the method.

Among the objects of the invention may be noted the provision of a method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum; the provision of such a method whereby the flexible coating obtained can be subjected to bending, forming, roll forming, drawing and similar operations without adversely atfecting the coating; the provision of such a method whereby flexible coatings of dilferent colors may be obtained by employing different aluminum alloys and varying the conditions of treatment; the provision of a method of the type described whereby the flexible coating obtained may be readily colored with organic or inorganic coloring materials; the provision of a method of this character which may be applied either by batch or continuous operation to previously formed aluminum articles or to coiled or strip aluminum or aluminum alloys; the provision of novel aluminum products coated in accordance with the above described method; and the provision of novel bath compositions for use in the anodic treatment of aluminum. Other objects and features will be in part apparent and in part pointed out hereinafter. v

The invention accordingly comprises the products and methods hereinafter described, the scope of the invention being indicated in the'following claims.

Many methods are known for electrolytically producing an oxide coatingon the surface of aluminum and its alloys. In general, these methods involve the anodictreatment of aluminum in various electrolytes in such a manner that a thin, inert coating of aluminum oxide is obtained on the metal surface. In anodizing, the aluminum is made the anode and an electrolyte capable of yielding oxygen uponelectrolysis is employed. An initial oxide coating is formed on the surface of the aluminum upon the passing of current through the electrolyte and further oxidation takes place beneath this coating as the electrolysis progresses. The aluminum oxide coating finally obtained is minutely porous and possesses good absorption qualities for dyes and paints 2,788,317 Patented Apr. 9, i957 In most instances, hard oxide coatings having good abrasive and corrosion resistance properties are obtained. However, insofar as is known, none of the methods heretofore utilized yields an oxide coated aluminum product which can be fabricated without injuring the coating. Thus, because of the brittleness of the oxide coating produced by these methods, it has not been possible, prior to the present invention, to carry out bending, forming, roll forming or drawing operations on the aluminum after the anodizing treatment. Whenever fabrication operations of this type have been attempted, it has been found that microscopic cracks occur in the oxide coating thereby allowing corrosive agents access to the aluminum base metal, resulting in deep corrosion and annulation. Although one method provides a product which can be subjected to a drawing operation, this process is unsatisfactory because it requires special treatments to be applied subsequent to anodizing in order to insure the corrosion resistance of the oxide coating.

In accordance with the present invention, it has now been found that a flexible, corrosion-resistant protective coating may be produced on aluminum and its alloys by anodically treatingthe aluminum in certain novel bath compositions. The coating obtained consists essentially of. aluminum oxide crystals interspersed with hydrated aluminum hydroxide. The method of this invention not only yields a coating which possesses excellent abrasive and corrosion resistance properties, but bending, forming, roll forming, drawing and similar fabrication operations can be carried out on the product without breaking the coating, and, accordingly, without substantially affecting these properties. Moreover, the method of the present invention results in an opaque coating which gives the aluminum the appearance of a lacquer or enamel finish rather than a metallic finish. Further, the coating may be conveniently colored according to conventional methods, if desired. Even though the coating is uniform, it is of sufficient porosity to absorb the necessary amount of color to insure excellent light resistance. The method of my invention may be carried out batchwise or continuously on previously. formed aluminum articles or may be applied to coiled or strip aluminum and its alloys. The flexible, corrosion-resistant protective coating of the present invention is electrolytically produced on aluminum and its alloys by anodically treating the aluminum in novel bath compositions which have been found to affect the structure and form of the crystals in the coating so as to render the coating flexible. The first step in my method consists in cleaning the aluminum to be treated according to any of the conventional Ways well known to the art. The aluminum is then made the anode in an electrolytic bath containing chromic acid, trivalent chromium and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate. The trivalent chromium may be formed in situ by reducing a portion of the chromic acid originally present in the bath. it is essential that the bath contain both trivalent and hexavalent chromium, and in addition, at least twoof the aforementioned metal dichromates. Further, the relative amounts of trivalent and hexavalent chromium must be so proportioned that the amount of trivalent chromium present in the bath does not exceed 5% by weight of the total chromium content of the bath, and preferably is between 0.4% and 5% by weight of the total chromium content. It is also essential that the bath be substantially free of combined or free sulfuric acid, i. e., both sulfuric acid and sulfates, and if the bath contains more than approximately 0.05 g./liter of sulfuric acid, the process will not operate satisfactorily.

The cathode in the electrolytic bath may be of any suitable material, but I prefer to employ a steel sheet for this purpose. it is preferred that the anodic treatment be carried out at a voltage between and volts direct or alternating current, but any voltage between 15 and volts direct or alternating current is satisfactory. With respect to temperature, this should be maintained between approximately C. and approximately 60? C. during the anodic treatment. The current density will vary with the temperature and the composition of the aluminum alloy being treated, an increase in temperature giving rise to an increase in the current density. In general, however, the current density is preferably less than 10 amperes per square foot.

During the anodic treatment,.,the. aluminum .dichro mate content of the bath is gradually built up by the reaction of the bathwith' the aluminum being trcated,.but, the bath may be used for as long as one year before.

it becomes advisable to replace the bath. As the aluminum dichromate content of the bath increases, the conductivity of the bath decreases and the thickness of the coating obtained. on the article being treated also decreases. Thus, the thickness of the coating can be measured inorder to determine whether or not the bath should be replaced. In addition, the iron dichromate content of the bath increases until the tank walls are passivated. A make-up solution corresponding to the bath composition is added periodically, not only to replace that portion of the bath which is gradually lost by being mechanically taken out of the tank, but also to add chromic acid to replace the amounts lost by reaction with the aluminum being treated andthe tank, walls.

As the electrolysis proceeds, a coating consisting essentially of aluminum oxide crystals interspersed with hydrated aluminum hydroxide is formed on the surfaces of the aluminum. It is preferred that the treatment be carried out for a period of approximately 25 to minutes so that a coating having a thickness of approximately 0.0003 inch is obtained. If. a deeper coating is desired, the treatment period may be extended up to 120 minutes, for example. However, the coating should not be allowed to attain too great a thickness since the surface thereof then becomes powdery. In the preferred form of the invention, the coating has a thickness between approximately 0.0002 inch and aproxirnately 0.00.06 inch. The coatings obtained through the method of the present invention are characterized by the fact that the coating is flexible, not brittle, and will not break or crack.

in the coating a dye or other soluble organic or inorganic coloring substance by treating the .aluminum with, a solution thereof. Notwithstanding the small dimensions or" the aluminum oxide crystals, the coatings possess sufiicient porosity to enable them to absorb the proper amount of color for the best light resistance.

After the anodic treatment and coloring operation, if.

any, have been completed the aluminum is subjected to the action of steam in order to seal the pores of the coating and render the coating corrosion-resistant. During thissealing treatment, the poresbecome closed through the formation of a hydrate of aluminum hydroxide. It is preferred in the practice of the invention 'that" sealing 4, be carried out using steam at atmospheric pressure. However, if the aluminum has not been colored with a dye, superheated steam may be used. The best sealing results are obtained when steam under a pressure of one atmosphere is employed for a period of from approximately 5 to 30 minutes.

The following'examples illustrate the invention;

Example 1 An electrolyte having the following compositionpwas employed as the bath:

Sodium stannate g Iron dichromate g 5 Aluminum dichromate g 5 Trivalent chromium (expressed as chromium) g 0.5 Chromic acid g 180 Water Balance to a liter The trivalent chromium was formed in situby'adding approximately 20 ml. of methyl alcoholtothe bath,

which contained a proportionately greater amount of' chromic acid. The tin bichromate was also formed in situ by reaction between -chromic acid "and sodium stanmate. The bathcontained less than 005g. of sulfuric acid.

The aluminum work was immersed in'the bath and was made the anode. A steel sheet acted as the cathode. A voltage of 23 volts direct current was applied to the bath. The temperature of the bath was maintained at 43 C. for a period of 45 minutes. At the end of this period, the aluminum work was withdrawn from the bath. The coating produced on the aluminum work was hard, corrosion-resistant and very flexible.

The coating was alight shade. of gray-white and was very hard and flexible. Best results are obtained when this bath composition is employed with pure aluminum and aluminum alloys with manganese and a smallquantity of silicon (up to 1%), Zinc and iron.

Example 2 An electrolyte having the following composition was employed as. the bath:

Zinc dichromate g 60 Iron dichromate; g 8 Trivalent chromium (expressed as chromium) g; 0.4 Chromic'acid g Water Balance to a liter The trivalent-chromium was formed in situas in:Examplel. Also, the bath contained less than 0.05 ,g. of sulfuric acid.

The. aluminunrwork was immersed in the bath and was.

A steel sheet acted as the cathode. A-

made the anode. voltage of 21 volts direct current was applied to. the bath. The temperature of the bath was maintained at 52 C. for a period of 45 minutes. At the end of this period, the aluminum work was withdrawn from the composition is especially suitable for use with aluminum alloys containing; magnesium.

Example 3 An electrolytev havingthe following composition-was employed as the bath:

Magnesium dichromate g 60. Aluminum dichromate g 6 Trivalent chromium (expressed as chromium) g 0.6 Chromic acid g 140 Watered u, Ba'lance-to a liter The trivalent chromium was formed in situ as in Example 1, and the bath contained less than 0.05 g. of sulfuric acid.

The aluminum work was immersed in the bath and was made the anode. A steel sheet acted as the cathode. A voltage of 25 volts direct current was applied to the bath. The temperature of the bath was maintained at 41 C. for a period of 45 minutes. At the end of this period, the aluminum work was withdrawn from the bath. The coating produced on the aluminum work was hard, corrosion-resistant and very flexible.

This bath composition gives excellent results with pure aluminum and aluminum alloys containing copper.

It will be understood that, although I prefer to form the trivalent chromium in situ by reduction of a proportion of the chromic acid, this is not essential. Furthermore, if desired, reducing agents other than methyl alcohol, for example, oxalic acid, or hydrogen peroxide may be employed.

The present invention not only affords a convenient method of producing a corrosion-resisting protective coating on aluminum and its alloys but this coating is flexible to a remarkable extent. Previously known protective coatings formed by electrolytic processes have been brittle to the extent that it was impractical to form articles from the treated aluminum. As a result, only the formed aluminum shape could be anodized and even then the resulting coating was of comparatively limited value. Even though the formed article was not subsequently subjected to forming operations or bending, blows experienced in usage would frequently open cracks in the coatmg.

The protective coatings of the present invention on the other hand are so flexible that not only may they be produced on formed articles but they may be produced on strips or coils of aluminum or its alloys from which such articles are subsequently manufactured. Since the anodized aluminum can be bent through 180 without cracking the protective coating, the usual aluminum articles may be formed in the customary manner from stock which already has the protective surface layer thereupon.

The term aluminum as employed herein includes aluminum per se and aluminum alloys, i. e., alloys containing more than 50% by weight of aluminum.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

i claim:

1. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting less than 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and tree sulfuric acid.

2. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichrornate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichrornate and titanium dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid, and thereafter sealing the pores of the coating thus formed.

3. The method of electroiytically producing a flexible, corrosion-resistant protective coating on aluminum con1- prising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichrornate, Zinc dichrornate, magnesium dichromate, aluminum dichrornate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximatel 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid, absorbing in the coating formed by the anodic treatment a soluble coloring substance by treating the aluminum with a solution of said substance, and thereafter subjecting the aluminum to the action of steam.

4. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially oi? chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichrornate, zinc dichromate, magnesium dichrornate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting between approximately 0.4% by Weight and approxiiately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid, the anodic treatment being carried out at a temperature of between approximately C. and approximately C. and a voltage of between approximately 15 and approximately 30 volts, and thereafter subjecting the aluminum to the action of steam.

5. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions, iron dichromate and aluminum dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid and the anodic treatment being carried out at a temperature of between approximately 40 C. and C. and at a voltage of between approximately 20 and 25 volts, and thereafter subjecting the aluminum to the action of steam at atmospheric pressure.

6. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions, iron dichromate and zinc dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid and the anodic treatment being carried out at a temperature of between approximately 35 C. and 58 C. and at a voltage of between approximately 20 and 25 volts, and thereafter subjecting the aluminum to the action of steam at atmospheric pressure.

7. The method of electrolytically producing a flexible, corrosion-resistant protective coating on aluminum comprising anodically treating the aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions, magnesium dichromate and aluminum dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid and the anodic treatment being carried'out at a temperature of between approximately 40 C. and 65 C. and at a voltage of between approximately 20 and 25 volts, and thereafter subjecting the aluminum to the action of steam at atmospheric pressure.

8. A bath composition for the anodic treatment of aluminum consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting less than 5% by Weight of the total chromium content of the bath composition and said bath composition being substantially free of combined and free sulfuric acid.

9. A bath composition for the anodic treatment of aluminum consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromatc and titanium dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath composition and said bath composition being substantially free of combined and free sulfuric acid.

10. A bath composition for the anodic treatment of aluminum consisting essentially of chromic acid, trivalent chromium ions, iron dichromate and aluminum dichromate, the trivalent chromium constituting approximately 2.8% by weight of the total chromium content of the bath composition and said bath composition being substantially free of combined and free sulfuric acid.

ll. A bath composition for the anodic treatment of aluminum consisting essentially of chromic acid, trivalent chromium ions, iron dichromate and zinc dichromate, the trivalent chromium constituting approximately 2.8% by weight of the total chromium content of the bath composition and said bath composition being substantially free of combined and free sulfuric acid.

12. A bath composition for the anodic treatment of aluminum consisting essentially of chromic acid, trivalent chromium ions, magnesium dichromate and aluminum (lichromate, the trivalent chromium constituting approximately 4% by Weight of the total chromium content of the bath composition and said bath composition being substantially free of combined and free sulfuric acid.

13. An aluminum product having on at least two surfaces thereof a relatively thin, flexible, corrosion-resistant, high-wearing coating having a thickness between approximatcly 0.0002 inch and approximately 0.001 inch, said coating being formed by anodically treating aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting less than approximately 5% of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid, and thereafter scaling the pores of the coating thus formed.

14. An aluminum product having on at least two surlaces thereof a relatively thin, flexible, corrosion-resistant, high-wearing coating said coating consisting essentially of aluminum oxide crystals interspersed with hydrated aluminum hydroxide and having a thickness of between approximately 0.0002 inch and approximately 0.0006 inch, said coating being formed by anodically treating aluminum in a bath consisting essentially of chromic acid, trivalent chromium ions and at least two compounds selected from the group consisting of tin dichromate, Zinc dichromate, magnesium dichromate, aluminum dichromate, iron dichromate, beryllium dichromate, zirconium dichromate and titanium dichromate, the trivalent chromium constituting between approximately 0.4% by weight and approximately 5% by weight of the total chromium content of the bath and the bath being substantially free of combined and free sulfuric acid, and thereafter subjecting the aluminum to the action of steam.

References Cited in the file of this patent UNlTED STATES P TENTS 1,977,622 Buzzard Oct. 23, 1934 2,150,395 Nagata Mar. 14, 1939 2,260,278 Schenk Oct. 21, 1941 2,356,543 Sonnino et al Aug. 22, 1944 2,665,243 Young et al Jan. 5, 1954 FOREIGN PATENTS 223,994 Great Britain Nov. 3, 1924 OTHER REFERENCES Metal Finishing, June 1941, pp. 301 and 302 of article by Mozley. 

1. THE METHOD OF ELECTROLYTICALLY PRODUCING A FLEXIBLE, CORROSION-RESISTANT PROTECTIVE COATING ON ALUMINUM COMPRISING ANODICALLY TREATING THE ALUMINUM IN A BATH CONSISTING ESSENTIALLY OF CHROMIC ACID, TRIVALENT CHROMIUM IONS AND AT LEAST TWO COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF TIN DICHROMATE, ZINC DICHROMATE, MAGNEISUM DICHROMATE, ALUMINUM DICHROMATE, ILRON DICHROMATE, BERYLLIUM DICHROMATE ZIRCONIUM DICHROMATE AND TITANIUM DICHROMATE, THE TRIVALENT CHROMIUM CONSTITUTING LESS THAN 5% BY WEIGHT OF THE TOTAL CHROMIUM CONTENT OF THE BATH AND THE BATH BEING SUBSTANTIALLY FREE OF COMBINED AND FREE SULFURIC ACID. 